1. Field of the Invention
This relates to the manufacture of 2-ethylhexanol. More particularly this invention relates to the manufacture of 2-ethylhexanol by aldol condensation of n-butyraldehyde, hydrogenation of the condensation product and recovery of 2-ethylhexanol in a multi-stage distillation workup from the hydrogenation product. The invention resides in the cracking of the distillation residue whereby to obtain cracked products, which can be beneficially hydrogenated in the hydrogenation stage and thus converted into additional 2-ethylhexanol.
2. Discussion of the Prior Art
2-Ethylhexanol is used in large quantities for the manufacture of dioctyl phthalate, the favored plastiziser for PVC. The synthesis and purification of the alcohol are carried out in three stages.
In the first stage of the known process, (c.f. Chemie-Ingenieur-Technik, 41, 1969, No. 17, p. 976) n-butyraldehyde is brought into intimate contact with an aqueous sodium hydroxide solution at an elevated temperature. The n-butyraldehyde is thereby converted into 2-ethylhexenal via aldolization and simultaneous cleavage of water. The applied temperatures should lie between 90.degree. and 130.degree. C. and the concentration of the sodium hydroxide solution should be between 1 to 5%. 2-Ethylhexenal, being insoluble in water, forms, together with the small amount of by-products produced, an upper layer in the aqueous alkaline solution which can be readily separted.
The following composition is typical of the product from this reaction stage:
95.0% 2-Ethylhexenal PA0 0.5% 2-Ethylhexanal PA0 0.1% 2-Ethylhexanol PA0 1.4% n-Butyraldehyde (monomer and trimer) PA0 3.0% higher condensation products
In the second stage, 2-ethylhexenal and 2-ethylhexanal are hydrogenated to 2-ethylhexanol, while n-butyraldehyde and portions of the trimeric n-butyraldehyde are hydrogenated to n-butanol. The higher condensation products containing the carbonyl group are hydrogenated to higher boiling compounds containing the hydroxyl group. The hydrogenation can take place with fixed bed hydrogenation catalysts in the liquid or gas phase. The pressures applied lie in the region of 20 to 200 bar for the liquid phase and generally between 2 to 10 bar for the gas phase. The temperatures employed are between 100.degree. and 180.degree. C.
Heptane occurs as by-product in very limited amounts, from the hydrogenation via decarbonylation of 2-ethylhexenal and 2-ethylhexanal. The recovery of pure 2-ethylhexanol in the third stage occurs distillatively via the separation of a fraction of the first runnings by means of a first running column and separation of a distillation residue in a main product column. Although the separation of the components with boiling points below that of 2-ethylhexanol can take place at normal or slightly reduced pressure, industrially, due to the high boiling point of 2-ethylhexanol, the entire distillation is carried out at a reduced pressure of 50 to 200 torr. However, it is not possible under these conditions to separate 2-ethylhexanol completely from the higher boiling components in the main product column. The bottom product of the main product column must therefore be redistilled in a further column, the last runnings column, in order to separate the 2-ethylhexanol in the residue from the higher boiling components. A temperature below 180.degree. C. is usually maintained in the bottom product with this distillation (last runnings distillation). At the head of the column, the remaining 2-ethylhexanol is obtained together with small amounts of the higher boiling substances as a last runnings fraction. This is recycled to the hydrogenation or distillation and is thereby subjected to a further distillation, so that finally, 2-ethylhexanol is almost completely separated from the higher boiling substances, which are removed at the bottom of the last runnings column. They consist of the reaction products of 2-ethylhexenal with itself or with n-butyraldehyde and constitute oxygen-containing compounds with more than 8 carbon atoms. The boiling points of the reaction products lie above that of 2-ethylhexanol.
The manufacture of 2-ethylhexanol takes place with very high yield. In the aldolization stage, relative to n-butyraldehyde, a conversion of over 98% and a selectivity of over 97% is obtained and in the hydrogenation stage a conversion of 100% and a selectivity of over 99% is reached. The possibility of directly increasing the yield of value products by improving the selectivity, within the framework of the present process, is therefore very limited.
However, attempts are still being made to further increase the yield of 2-ethylhexanol, as even minor improvements can result in significant economic benefits due to the large quantities in which 2-ethylhexanol is manufactured.
It is an object of this invention, therefore, to improve the yield of 2-ethylhexanol whereby to provide such significant economic benefits. It is a further object of this invention to increase the 2-ethylhexanol yield without an accompanying large expense and to thereby provide an improved, commercially feasible, and economic process for the realization of large quantities of 2-ethylhexanol. It is a further object of this invention to improve the yield of 2-ethylhexanol by converting the distillation residue, normally obtained in the manufacture of 2-ethylhexanol, into products which can be hydrogenated in the hydrogenation stage to form additional 2-ethylhexanol.